An organic electroluminescence (EL) device is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed.
An organic EL device is provided with a pair of electrodes formed of an anode and a cathode and an organic thin film layer between them. The organic thin film layer is a stacked body of layers having their respective functions. For example, the organic thin film layer is a stacked body of an anode, a hole-injecting layer, a hole-transporting layer, an emitting layer, a blocking layer, an electron-transporting layer and an electron-injecting layer in this sequence.
An organic EL device can be classified into two types, i.e. a fluorescent device and a phosphorescent device according to its emission principle. In a fluorescent organic EL device, emission derived from singlet excitons is used, and in a phosphorescent organic EL device, emission derived from triplet excitons is used. It is known that, in a phosphorescent device, in order to prevent diffusion of triplet excitons which have a longer exciton life than singlet excitons to the outside of an emitting layer, a material having a high triplet energy is used in a layer which is adjacent to the interface on the cathode side of the emitting layer, whereby a high efficiency is attained.
Patent Document 1 discloses a technology in which a blocking layer comprising BCP (bathocuproine), which is a phenanthroline derivative, is provided in adjacent to an emitting layer, whereby efficiency is increased by confining triplet excitons. Further, in Patent Document 2, in order to increase efficiency and prolong the lifetime, a specific aromatic ring compound is used in a hole-barrier layer.
On the other hand, in a fluorescent organic EL device, emission derived from triplet excitons has recently been reported (for example, Non-Patent Documents 1 and 2, Patent Document 3).